Various anti-cancer drugs (a.k.a., antineoplastic agents) are widely used for treatment of both tumorous and leukemic parental cancer cells. Certain cell lines, however, are multi-drug resistant (MDR) and many standard anti-cancer drugs are MDR substrates. MDR, a phenomenon characterized by over-expression of the mdr1 gene product P-glycoprotein and cross-resistance to multiple classes of anticancer drugs, including quinones, seriously complicates and compromises the effective chemotherapy of cancer. For example, as compared to their parental counterparts, many MDR cell lines are over 10-fold more resistant to the well known antineoplastic agents doxorubicin (DOX) and etoposide (ETP). Accordingly, there is a constant search for antineoplastic agents that are not MDR substrates and which are therefore capable of effectively treating MDR cancers.
Naturally occurring substances are a promising source for such agents. Blackseed, the seed of Nigella sativa L. [Ranunculaceae], has been employed for thousands of years as a spice and food preservative, as well as a protective and curative remedy for numerous disorders. The historical tradition of blackseed in medicine is substantial. It is the blackseed referred to by the prophet Mohammed as having healing powers; blackseed is also identified as the curative black cumin in the Holy Bible, and is described as the Melanthion of Hippocrates and Doscorides and as the Gith of Pliny. Several beneficial pharmacological effects have been attributed to various crude and purified components of blackseed, including antihistaminergic, antihypertensive, hypoglycemic, antimicrobial, mast cell stabilizing and anti-inflammatory activities.
Blackseed preparations may have significant in vitro and in vivo antineoplastic activity. A crude methanolic extract of Nigella sativa seed has an in vitro IC.sub.50 to Erlich ascites carcinoma, Dalton's ascites lymphoma and sarcoma 180 cells of 1.5, 3 and 1.5 .mu.g/mL, respectively, while exerting minimal cytotoxicity to normal lymphocytes. This extract is also active in vivo, completely inhibiting the growth of Erlich ascites carcinoma in mice. Exposure to the volatile oil obtained from blackseed is believed to alter the cellular expression of specific polypeptides in Jurkart T lymphoma cells, suggesting that changes in polypeptide expression might play a role in the biological activities attributed to blackseed. In addition to these direct anti-tumor effects, blackseed preparations may have potential for cancer chemoprevention, as well as for reducing the toxicity of standard antineoplastic drugs. Topical application of a blackseed extract can inhibit the two-stage initiation-promotion (by dimethylbenz[a]anthracene-croton oil) of skin carcinogenesis in mice. Intraperitoneal injections of the same blackseed extract may reduce the incidence of soft tissue sarcomas observed after 30 days of subcutaneous 20-methylcholanthrene (MCA) injections by 67% as compared to MCA-treated controls. Another chemoprotective effect is treatment with a crude ethanolic extract of blackseed, which may significantly reduce cisplatin-induced leukopenia and hemoglobinemia in cisplatin-treated mice.
Although the above applications illustrate the antineoplastic potential of crude blackseed preparations, they do not establish the anti-cancer activity of specific blackseed components, in particular that of the quinones thymoquinone (TQ) and dithymoquinone (DIM). Further, no studies have been reported which evaluate the cytotoxicity of the Nigella sativa seed components TQ and DIM for multi-drug resistant cancer cells. Thus, comprehensive cytotoxicity assays of these potential anticancer drugs in both parental and MDR tumor cell lines are useful for a thorough evaluation and rational therapeutic development of these agents.